Gas discharge tube

ABSTRACT

A gas discharge tube  10  of the present invention generates an electric discharge between an anode part  24  and a cathode part  56  disposed inside a sealed container  12  in which gas has been contained. The gas discharge tube  10  comprises an electric discharge path restricting part  28  which is cylindrical and disposed between the anode part and the cathode part, and which has a throughhole  42  for narrowing an electric discharge path between the anode part and the cathode part; and an electric discharge shielding part  50  which is disposed to surround the electric discharge path restricting part so as to restrict an electric discharge from the outer peripheral surface of the electric discharge path restricting part to the cathode part, and which is electrically isolated from the electric discharge path restricting part. The end part of the electric discharge path restricting part on the cathode part side projects beyond a surface of the electric discharge shielding part on the cathode side by a predetermined amount “P” of projection. With this structure, a high density electron region is formed exclusively in a part of the throughhole of the electric discharge path restricting part on the cathode side, thereby ensuring the generation of a startup discharge.

TECHNICAL FIELD

The present invention relates to a gas discharge tube such as a heavyhydrogen lamp to be used particularly as a light source forspectroscopy, chromatography, etc.

BACKGROUND ART

As conventional techniques in the above-described field, those disclosedin Japanese Unexamined Patent Publication No. H7-288106 and JapaneseUnexamined Patent Publication No. H10-64479 are known. In either gasdischarge tube, a barrier wall made of metal is disposed on an electricdischarge path between an anode part and a cathode part, and a smallhole is formed on the barrier wall so as to narrow the electricdischarge path. In such a structure, light with high brightness can beobtained by the small hole on the electric discharge path. Inparticular, in the gas discharge tube of Japanese Unexamined PatentPublication No. H7-288106, brightness is further increased by extendingthe length of the small hole, that is, a portion of the electricdischarge path that is narrowed. On the other hand, in the gas dischargetube of Japanese Unexamined Patent Publication No. H10-64479, higherbrightness is achieved by disposing a plurality of barrier walls inaddition to extending the length of the small hole.

The demand for higher brightness in the technical field of gas dischargetubes has been comparatively satisfied by the techniques disclosed inthe above-described patent publications.

However, when the narrowed portion of the electric discharge path isextended in length, an electric discharge is less liable to occur. Toavoid this problem, in the gas discharge tube disclosed in the abovesecond patent publication, a plurality of metal barrier walls aredisposed to generate an electric discharge step by step; however, thereis a problem in that this complicates a power supply circuit.

Therefore, an object of the present invention is to provide a gasdischarge tube which can securely generate an electric discharge,regardless of the length of a portion of the electric discharge paththat is narrowed.

DISCLOSURE OF THE INVENTION

In order to achieve the above-mentioned object, the present inventionprovides a gas discharge tube comprising: a sealed container in whichgas is contained; an anode part disposed in the sealed container; acathode part defining an electric discharge part for generating anelectric discharge with the anode part, the cathode part being disposedinside the sealed container in such a manner as to be distanced from theanode part; an electric discharge path restricting part beingcylindrical and conductive, and having a throughhole for narrowing theelectric discharge path, the electric discharge path restricting partbeing disposed between the anode part and the cathode part, and beingadapted to be electrically connected with an external power source; andan electric discharge shielding part disposed to enclose the electricdischarge path restricting part and electrically isolated from theelectric discharge path restricting part, wherein the electric dischargepath restricting part and the electric discharge shielding part arepositioned in such a manner that an end part of the electric dischargepath restricting part on the cathode part side projects beyond a surfaceof the electric discharge shielding part on the cathode part side by apredetermined amount. It is preferable that the amount of projection beapproximately 0.5 mm at maximum.

With this arrangement, most of the electric discharge path extendingfrom the outer peripheral surface of the electric discharge pathrestricting part to the cathode part is shielded by the electricdischarge shielding part. Furthermore, only a portion of the end part ofthe electric discharge path controlling part that is on the cathode partside, or a portion with a maximum of approximately 0.5 mm as an amountof projection forms an electric discharge path for a startup dischargewith the cathode part. Consequently, when electric power for startup isturned on, a high density electron region is formed exclusively in thevicinity of a projecting tip portion of the electric discharge pathrestricting part and a portion of the throughhole that is on the cathodepart side. This ensures the generation of a startup discharge.

Preferably, the throughhole in the electric discharge path restrictingpart comprises a small hole part which is provided on the anode partside and which has a constant inner diameter, and an increased diameterhole part which is linked with the small hole part and extends towardthe cathode part side while increasing in diameter toward the cathodepart side in a funnel shape. The small hole part functions as a portionfor narrowing the electric discharge path, and the increased diameterhole part forms an excellent arc ball inside, thereby contributing tohigher brightness.

By making the increased diameter hole part in the electric dischargepath restricting part have an inner peripheral surface which extendsbeyond a surface of the electric discharge shielding part that is on thecathode part side toward a surface of the electric discharge shieldingpart that is on the anode part side, the formation of the high densityelectron region is concentrated particularly inside the increaseddiameter hole part. Therefore, further ensuring the generation of thestartup discharge. When the inner diameter of the small hole part in theelectric discharge path restricting part is made D1 and the maximuminner diameter of the increased diameter hole part is made D2, it iseffective to set D2 in the range of 1 mm to 3 mm, and to set the ratioD2/D1 in the range of 4 to 10 so as to achieve a higher density electronregion and an excellent arc ball formation.

It is also preferable that the electric discharge shielding part be madeof electrically insulating material in order to easily provide electricisolation with the electric discharge path restricting part.

The above-mentioned object, other features and advantages of the presentinvention will be made clear to those skilled in the art through afollowing detailed description with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an end view showing a first embodiment of a gas discharge tubeof the present invention.

FIG. 2 is an enlarged cross sectional view of a vicinity of an electricdischarge path restricting part in the gas-discharge tube shown in FIG.1.

FIG. 3 is an end view showing a modified example of the gas dischargetube of the first embodiment.

FIG. 4 is an enlarged cross sectional view of an electric discharge pathrestricting part in the gas discharge tube shown in FIG. 3.

FIG. 5 is a cross sectional view showing another modified example of thevicinity of the electric discharge path restricting part.

FIG. 6 is an end view showing a second embodiment of the gas dischargetube of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Now, preferable embodiments of the gas discharge tube of the presentinvention will be described in detail with reference to accompanyingdrawings. In the following description, it is to be understood thatvarious terms indicating directions such as “upeardly”, “dowinwardly”and the like are referred to based on the conditions of correspondingdrawings for the sake of convenience, and should not be construed aslimiting terms.

FIRST EMBODIMENT

FIG. 1 shows an end view in a condition where a first embodiment of thegas discharge tube of the present invention is cut in the directionorthogonal to the axis (tube axis). A gas discharge tube 10 shown inFIG. 1 is a side-on type heavy hydrogen lamp, and has a sealed container12 made of glass in which several hundreds of Pa of heavy hydrogen gashas been sealed. The sealed container 12 comprises a side tube part 14which is cylindrical and sealed at one end thereof, and a stem part (notshown) for sealing the other end of the side tube part 14. A portion ofthe side tube part 14 is used as a light emitting window 18. The sealedcontainer 12 accommodates a light emission part assembly 20 therein.

The light emission part assembly 20 includes a base part 22 which iselectrically insulating made of ceramics or the like. The base part 22is disposed opposite the light emitting window 18, and has a concavepart 23 on its upper surface. Over the base part 22 is formed aplate-shaped anode part 24. Onto the rear side of the anode part 24, atip portion of a stem pin 26 is fixedly connected electrically. The tipportion extends in the direction of the tube axis (the center axis ofthe side tube part 14), and stands on the stem part.

The light emission part assembly 20 also has anelectric-discharge-path-restricting-part supporting part (hereinafterreferred to as supporting part) 30 for supporting an electric dischargepath restricting part 28 that will be described later. The supportingpart 30 is tabular and made of ceramics or the like so as to beelectrically insulating, and is fixed on the top end surface of theouter peripheral part of the base part 22. The supporting part 30 has aconcave part 32 on a bottom surface center thereof. The bottom surface(downward surface) and side surfaces of the concave part 32 aredistanced from the anode part 24 by a predetermined spacing. Thesupporting part 30 has an opening 34 in its center.

Furthermore, a conductive plate 36 is provided along the bottom and sidesurfaces of the concave part 32 of the supporting part 30 in such amanner as to be in contact with these surfaces. The conductive plate 36is electrically connected with the tip portion of a stem pin 38 standingon the stem part. The conductive plate 36 has an opening 40 in itscenter, which is disposed coaxially with the opening 34 of thesupporting part 30 when the conductive plate 36 is applied to thesupporting part 30. The inner diameter of the opening 40 is designed tobe smaller than the inner diameter of the opening 34.

In the center of the upper surface of the conductive plate 36, anelectric discharge path restricting part 28 made of conductive materialsuch as metal (e.g., molybdenum, tungsten, or an alloy of these) isfixedly welded in such a manner as to be coaxial with the openings 34and 40. This makes it possible to feed electric power from outside tothe electric discharge path restricting part 28 via the conductive plate36 and the stem pin 38.

As shown in FIG. 2, the electric discharge path restricting part 28 isshaped like a cylinder having a throughhole 42 inside for narrowing theelectric discharge path, and is provided with a flange part 44 forfixing at the end part on the anode part 24 side. The outer diameter ofthe flange part 44 is substantially equal to the inner diameter of theopening 34 of the supporting part. 30. Therefore, after the flange part44 is inserted into the opening 34 and the bottom surface of the flangepart 44 is brought into contact with the upper surface of the conductiveplate 36, the flange part 44 is fixed to the conductive plate 36 bywelding or the like, thereby making the supporting part 30 support theelectric discharge path restricting part 28. Since the electricdischarge path restricting part 28 has a convex-shaped side surface withthe flange part 44, the top end portion projecting toward the cathodeside can be reduced, thereby contributing to ensuring a startupdischarge which will be described later.

The throughhole 42 of the electric discharge path restricting part 28 isformed of a small hole part 46 which is provided on the anode part 24side in such a manner as to have a constant inner diameter, and of anincreased diameter hole part 48 which is linked with the small hole part46 and extends upward while increasing in diameter in a funnel shape.The small hole part 46 is mainly for narrowing the electric dischargepath, and the increased diameter hole part 48 is mainly for forming anarc ball, and in the present embodiment, has a cone-shaped innerperipheral surface. In order to narrow the electric discharge path, itis preferable that the small hole part 46 has an inner diameter D1 of0.5 mm or so. In addition, it is preferable that a maximum innerdiameter D2 of the increased diameter hole part 48, that is, the innerdiameter D2 of the throughhole 42 on the end surface on the cathode sidebe in the range of 1 mm to 3 mm, and it is further preferable that D2/D1or the ratio of the inner diameter D2 to the diameter D1 of the smallhole part 46 is in the range of 4 to 10.

An electric discharge shielding part 50 with a tabular shape is disposedon the upper surface (the surface on the cathode side) of the supportingpart 30 in such a manner as to be in contact with these surfaces. Inthis first embodiment, the electric discharge shielding part 50 is madeof conductive material such as metal. The electric discharge shieldingpart 50 has an opening 52, and the electric discharge shielding part 50is positioned with respect to the supporting part 30 in such a mannerthat the opening 52 is coaxial with the opening 34 of the supportingpart 30. As shown in FIG. 2, the opening 52 of the electric dischargeshielding part 50 has an inner diameter “d” which is slightly largerthan an outer diameter D3 of the cylindrical part (the portion upperthan the flange part 44) 54 of the electric discharge path restrictingpart 28. In an assembled condition, the cylindrical part 54 of theelectric discharge path restricting part 28 is inserted in the opening52 of the electric discharge shielding part 50 such that the electricdischarge shielding part 50 surrounds the cylindrical part 54. Aclearance, which is formed between the inner peripheral surface of theopening 52 of the electric discharge shielding part 50 and the outerperipheral surface of the cylindrical part 54 of the electric dischargepath restricting part 28, is made small so as to cause slight orsubstantially no leakage of an electric discharge passing through theclearance. The presence of the clearance provides electric isolationbetween the electric discharge shielding part 50 attached to thesupporting part 30 that is electrically insulating and the electricdischarge path restricting part 28, and also puts the electric dischargeshielding part 50 out of contact with other parts to be applied with apotential and hence in a potentially floating state.

The whole length (height) “H” of the electric discharge path restrictingpart 28 is slightly larger than a sum “T” of the thicknesses of thesupporting part 30 and the electric discharge shielding part 50, so thatthe top end of the electric discharge path restricting part 28 projectsupward beyond the upper surface of the electric discharge shielding part50. The amount of projection “P” is approximately 0.5 mm at maximum, andpreferably 0.3 mm. A length “h” of the increased-diameter hole part 48,which is a cathode-side portion of the throughhole 42 in the electricdischarge path restricting part 28, is larger than the amount ofprojection “P.” In other words, the bottom end of the increased diameterhole part 48 (the border between the increased diameter hole part 48 andthe small hole part 46) is closer to the anode part 24 than the uppersurface of the electric discharge shielding part 50.

The light emission part assembly 20 also includes a cathode part 56which is disposed outside the light path on the light emission window 18side. The cathode part 56 is provided for generating thermal electrons,and more specifically, is formed by coating electron emitting materialonto a coil which is extended in the tube axial direction and is made oftungsten. The cathode part 56 is electrically connected with the tipportion of an unillustrated stem pin standing on the stem part via aconnection pin so as to allow feeding of electric power from outside.

The light emission part assembly 20 also includes an electric dischargedistributor 58 made of metal and a front surface cover 60 in order toprevent materials spattered or evaporated from the cathode part 56 fromadhering to the light emission window 18. The electric dischargedistributor 58 is disposed to surround the cathode part 56 and is fixedon the upper surface of the supporting part 30. The front surface cover60 is opposite the electric discharge distributor 58 and is fixed on theupper surface of the supporting part 30. Between the electric dischargedistributor 58 and the front surface cover 60, a light passage opening62 for letting discharge light pass through is formed. The electricdischarge distributor 58 has an opening 64 formed in a portion thatfaces the front surface cover 60, and thermal electrons generated in thecathode part 56 pass through the opening 64.

Now, operations of the above-mentioned gas discharge tube 10 will bedescribed.

First, before an electric discharge, for 20 seconds or so, electricpower of approximately 10 W is supplied to the cathode part 56 from acathode external power source (not shown) via a stem pin (not shown) soas to preheat a coil composing the cathode part 56. Next, a voltage ofapproximately 160V is applied between the cathode part 56 and the anodepart 24 from a main discharge external power source (not shown) via thestem pin 26, thereby preparing an arc discharge.

Later, a predetermined voltage, e.g. approximately 350V is appliedbetween the electric discharge path restricting part 28 and the anodepart 24 via the stem pins 38 and 26 from a trigger external power source(not shown). As a result, a startup discharge occurs between the cathodepart 56 and a projecting portion of the electric discharge pathrestricting part 28 that is closer to the cathode part 56 than the uppersurface of the electric discharge shielding part 50.

Here, in this embodiment, most of the electric discharge path from theouter surface of the electric discharge path restricting part 28 to thecathode part 56 is shielded by the electric discharge shielding part 50,and an electric discharge path for a startup discharge is formed withthe cathode part 56 only in the projecting portion on the top end of thecylindrical part 54 of the electric discharge path controlling part 28,that is, the portion corresponding to the amount of projection “P” of0.5 mm at maximum, and preferably 0.3 mm. Consequently, a high densityelectron region is formed exclusively in and around the increaseddiameter hole part 48 of the electric discharge path restricting part28. Furthermore, since the cone-shaped inner peripheral surface of theincreased diameter hole part 48 is extended further downward than theupper surface of the electric discharge shielding part 50, the highdensity electron region is formed particularly inside the increaseddiameter hole part 48. This ensures the generation of a startupdischarge.

The occurrence of a startup discharge between the top end part of theelectric discharge path restricting part 28 and the cathode part 56 isfollowed by the occurrence of a startup discharge between the cathodepart 56 and the anode part 24, and later, a main discharge (arcdischarge) is generated due to a main discharge external electrode. Sucha gradual generation of an electric discharge ensures the generation ofa main discharge even when the whole length “H” of the electricdischarge path restricting part 28 is made large enough for narrowingthe electric discharge path (e.g. 2 mm or larger).

After generation of a main discharge, the electric power from thecathode external power source is adjusted to optimize the temperature ofthe cathode part 56. This maintains the main discharge between thecathode part 56 and the anode part 24, and forms an arc ball inside theincreased diameter hole part 48 of the electric discharge pathrestricting part 28. Since the electric discharge path is narrowed witha sufficient length in the electric discharge path restricting part 28and the arc ball is formed, ultraviolet rays to be generated arereleased outside, as light with extremely high brightness, after passingthrough the light emission window 18 of the sealed container 12 from thelight passage opening 62 between the electric discharge distributor 58and the front surface cover 60. The inner peripheral surface of theincreased diameter hole part 48 is cone-shaped; the inner diameter D2 ofthe increased diameter hole part 48 at maximum is in the range of 1 mmto 3 mm and the relationship D2/D1 with the inner diameter D1 of thesmall hole part 46 is made in the range of 4 to 10, so that the arc ballis formed in a stable and excellent shape. Consequently, the brightnessand amount of light to be emitted becomes stable. Making D1 and D2 theabove-mentioned size can further stimulate an increase in density of theelectron region in the increased diameter hole part 48.

FIG. 3 shows a modification of the gas discharge tube 10 shown in FIG. 1and FIG. 2. The gas discharge tube 110 shown in FIG. 3 differs from thegas discharge tube 10 shown in FIG. 1 and FIG. 2 in that an electricdischarge shielding part 150 is made of electrically insulating materialsuch as ceramics. The gas discharge tube 110 is substantially the sameas the gas discharge tube 10 in other aspects, so that components thesame as or equivalent to those in FIG. 1 and FIG. 2 are referred to withthe same reference numbers and overlapping description is omitted.

In the gas discharge tube 110 shown in FIG. 3, as described above, theelectric discharge shielding part 150 is made of electrically insulatingmaterial such as ceramics, so that even when it is in contact with theelectric discharge path restricting part 28, an electric discharge canbe shielded. This makes it easy to provide electrical isolation of theelectric discharge shielding part 150 from the electric discharge pathrestricting part 28 even when the positional precision between theelectric discharge path restricting part 28 and the electric dischargeshielding part 150 is low, thereby facilitating the manufacture.Furthermore, in this modification, as clearly shown in FIG. 4, the innerdiameter of the opening 152 of the electric discharge shielding part 150is nearly equal to the outer diameter of the cylindrical part 54 of theelectric discharge path restricting part 28, thereby causing noclearance between the electric discharge shielding part 150 and theelectric discharge path restricting part 28. Consequently, the electricdischarge path between the outer peripheral surface of the electricdischarge path restricting part 28 below the electric dischargeshielding part 150 and the cathode part 56, has a high shielding effect,and electrons have a higher density inside the increased diameter holepart 48 of the electric discharge path restricting part 28, therebysecuring the generation of a main discharge from a startup discharge.

As shown in FIG. 5, the electric discharge shielding part 150′ can beformed integral with the supporting part 130. This is because both ofthem are made of electrically insulating material such as ceramics. Suchan integral formation can reduce the number of components and facilitatethe manufacture.

SECOND EMBODIMENT

FIG. 6 is an end view showing a second embodiment of the gas dischargetube of the present invention cut along the axial direction. A gasdischarge tube 210 is a head-on type heavy hydrogen lamp, and has asealed container 212 made of glass in which several hundreds of Pa ofheavy hydrogen gas has been sealed. The sealed container 212 comprises aside tube part 214 which is cylindrical, a stem part 216 for sealing thebottom end side of the side tube part 214, and a light emission window218 for sealing the top end side of the side tube part 214. The sealedcontainer 212 accommodates a light emission part assembly 220.

The light emission part assembly 220 includes a base part 222 which istabular and made of ceramics or the like to be electrically insulating.The base part 222 is disposed opposite the light emitting window 218.Over the base part 212 is formed an anode part 224. With the anode part224, a tip portion of a stem pin (not shown) extending in the directionof the tube axis (the center axis of the side tube) standing on the stempart 216 is connected electrically.

The light emission part assembly 220 also has anelectric-discharge-path-restricting-part supporting part (supportingpart) 230, which is made of ceramics or the like to be electricallyinsulating. The supporting part 230 is disposed and fixed onto the uppersurface of the base part 222. In the center of the supporting part 230,a circular opening 234 is formed, into which the main portion (theportion shown in FIG. 6) of the anode part 224 is accommodated. In acondition where the main portion of the anode part 224 is disposed inthe opening 234 and the supporting part 230 is laid and fixed onto thebase part 222, an unillustrated end part of the anode part 224 issandwiched between the supporting part 230 and the-base part 222.

In addition, on the upper surface of the supporting part 230 is disposeda conductive plate 236 in such a manner as to be in contact with thesesurfaces. The conductive plate 236 is electrically connected with thetip portion of the stem pin 238 standing on the stem part 216. The stempin 238, and the above-mentioned stem pin connected with the anode part224 are wrapped with an electrically insulating tube 239 made ofceramics or the like so as not to be exposed between the stem part 216and the base part 222.

The conductive plate 236 has a circular opening 240 provided therein.The opening 240 has an inner diameter smaller than the inner diameter ofthe opening 234 of the supporting part 230. The opening 240 is disposedto be coaxial with the opening 234 of the supporting part 230 in acondition where the conductive plate 236 is fixed to the supporting part230.

In the center of the upper surface of the conductive plate 236, anelectric discharge path restricting part 228 made of metal for narrowingor restricting the electric discharge path from the anode part 224 isfixedly welded in such a manner as to be coaxial with the openings 234and 240. This enables electric power to be fed to the electric dischargepath restricting part 228 from outside via the conductive plate 236 andthe stem pin 238.

The electric discharge path restricting part 228 is substantiallyequivalent to the electric discharge path restricting part 28 of thefirst embodiment, that is, the one shown in FIG. 2. Therefore, when itis briefly described with the same reference numbers and with referenceto FIG. 2, the electric discharge path restricting part 228 is formed ofthe cylindrical part 54 and the flange part 44, and has inside thethroughhole 42 formed of the small hole part 46 and the increaseddiameter hole part 48.

The light emission part assembly 220 is further provided with adisc-shaped supporting part 270 for supporting an electric dischargeshielding part 250 which will be described later. The supporting part270 is made of electrically insulating material such as ceramics, and isdisposed on the upper surface of the supporting part 230 in such amanner as to be in contact with these surfaces. The supporting part 270has an opening 272 in its center for receiving the electric dischargepath restricting part 228 therethrough.

The electric discharge shielding part 250 is a conductive disc made ofmetal or the like and is disposed on the upper surface of the supportingpart 270 in such a manner as to be in contact with these surfaces. Theelectric discharge shielding part 250 has an opening 252 in its center,which is made coaxial with the opening 272 of the supporting part 270when in an assembled condition. The whole length “H” of the electricdischarge path controlling part 228 is slightly larger than a sum “T” ofthe thicknesses of the supporting part 270 and the electric dischargeshielding part 250, so that in an assembled condition, the top end ofthe electric discharge path restricting part 228 projects upward beyondthe upper surface of the electric discharge shielding part 250 by anamount of projection “P” of approximately 0.5 mm at maximum, andpreferably approximately 0.3 mm, while passing through the opening 252of the electric discharge shielding part 250. The amount of projection“P” is smaller than the length “h” of the increased diameter hole part48 of the electric discharge path restricting part 228, and the bottomend of the increased diameter hole part 48 is located lower than theupper surface of the electric discharge shielding part 250. Furthermore,the inner diameter of the opening 252 is slightly larger than the outerdiameter of the cylindrical part 54 of the electric discharge pathrestricting part 228, thereby forming a small clearance between them.The clearance lets the electric discharge shielding part be isolatedfrom the electric discharge path restricting part 228 and other parts tobe applied with a potential. This clearance enables substantialdischarge shielding.

The light emission part assembly 220 also includes a cathode part 256which is disposed outside the light path on the light emission window218 side. The cathode part 256 is provided for generating thermalelectrons, and to be more, specific, is formed by coating electronemitting material onto a coil which is extended in the tube axialdirection and is made of tungsten. The cathode part 256 is electricallyconnected with the tip portion of a stem pin (not shown) standing on thestem part 216 via a connection pin so as to allow feeding of electricpower from outside.

The light emission part assembly 220 further includes an electricdischarge distributor 258 made of metal and a front surface cover 260 inorder to prevent matter spattered or evaporated from the cathode part256 from adhering to the light emission window. The electric dischargedistributor 258 is disposed to surround the cathode part 256 and fixedon the upper surface of the supporting part 230. The front surface cover260 is opposite the electric discharge distributor 258 and is fixed onthe upper surface of the supporting part 230. Between the electricdischarge distributor 258 and the front surface cover 260, a lightpassage opening 262 for letting discharge light pass through is formed.The electric discharge distributor 258 has an opening 264 in a portionthat faces the front surface cover 260, and thermal electrons generatedin the cathode part 256 pass through the opening 264.

The gas discharge tube 210 according to the second embodiment thusstructured has the electric discharge path restricting part 228 and theelectric discharge shielding part 250 which are substantially the sameas their equivalents in the first embodiment, although there is adifference between a head-on type and a side-on type. In addition, thegas discharge tube 210 does not differ from the gas discharge tube 10 insize and positional relation, thereby bringing about an effect ofsecuring generation of a startup discharge and a main discharge.Furthermore, since the arc ball has a stable, excellent shape, theemitted light has high brightness, sufficient light amount andstability. A detailed description of operations of the gas dischargetube 110 will be omitted because it is similar to that of theabove-described gas discharge tube 10.

The electric discharge shielding part 250 in the gas discharge tube 210according to the second embodiment is made of conductive material suchas metal; however, it may also be made of electrically insulatingmaterial such as ceramics, and in that case, those skilled in the artwill understand easily that the structures shown in FIG. 3 to FIG. 5 canbe formed as modifications of the first embodiment.

As described above, the gas discharge tube of the present invention hasan effect of obtaining high brightness because of the provision of theelectric discharge path restricting part for sufficiently narrowing anelectric discharge path. The gas discharge tube has another effect ofsecuring a main discharge because the positional relation between theelectric discharge path restricting part and the electric dischargeshielding part ensures the generation of a startup discharge at the tipportion of the electric discharge path restricting part, therebyadvancing the startup discharge step by step.

As there is no need for providing a complicated power supply circuit,the cost in the whole device using the gas discharge tube of the presentinvention can be reduced.

Although the present invention and its advantages can be understoodhereinbefore, it is obvious that the above-described embodiments areonly typical preferable embodiments, and various modifications can becarried out in shape, structure and arrangement, without deviating fromthe spirit and scope of the present invention or losing the substantialadvantages.

1. A gas discharge tube comprising: a sealed container in which gas iscontained; an anode part disposed in said sealed container; a cathodepart defining an electric discharge part for generating an electricdischarge with said anode part, said cathode part being disposed insidesaid sealed container in such a manner as to be distanced from saidanode part; an electric discharge path restricting part beingcylindrical and conductive and having a throughhole for narrowing saidelectric discharge path, said electric discharge path restricting partbeing disposed between said anode part and said cathode part, and beingadapted to be electrically connected with an external power source; andan electric discharge shielding part disposed to enclose said electricdischarge path restricting part, said electric discharge shielding partbeing electrically isolated from said electric discharge pathrestricting part, wherein said electric discharge path restricting partand said electric discharge shielding part are positioned in such amanner that an end part of said electric discharge path restricting parton said cathode part side projects beyond a surface of said electricdischarge shielding part on said cathode part side by a predeterminedamount of projection.
 2. The gas discharge tube according to claim 1,wherein said amount of projection is approximately 0.5 mm at maximum. 3.The gas discharge tube according to claim 1, wherein said throughhole insaid electric discharge path restricting part includes a small hole partwhich is provided on said anode part side and has a constant innerdiameter, and a increased diameter hole part which is linked with saidsmall hole part and extends toward said cathode part side whileincreasing in diameter toward the cathode part side in a funnel shape.4. The gas discharge tube according to claim 3, wherein said increaseddiameter hole part in said electric discharge path restricting part hasan inner peripheral surface which extends beyond a surface of saidelectric discharge shielding part that is on said cathode part sidetoward a surface of said electric discharge shielding part that is onsaid anode part side.
 5. The gas discharge tube according to claim 3,wherein said increased diameter hole part has a maximum inner diameter(D2) in a range of 1 mm to 3 mm, and a ratio (D2/D1) of said maximuminner diameter (D2) of said increased diameter hole part to an innerdiameter (D1) of said small-hole part in said electric discharge pathrestricting part is in a range of 4 to
 10. 6. The gas discharge tubeaccording to claim 1, wherein said electric discharge shielding part ismade of electrically insulating material.
 7. The gas discharge tubeaccording to claim 1, wherein the gas in said sealed container is heavyhydrogen gas.